Use host PID namespace in docker example (#672)

* Use host PID namespace in docker example

See https://github.com/prometheus/node_exporter/issues/671

* Update readme for readability

* Fix comments in readme

README.md

@@ -166,40 +166,16 @@ To see all available configuration flags:
 ## Using Docker
 The node\_exporter is designed to monitor the host system. It's not recommended
 to deploy it as Docker container because it requires access to the host system.
-If you need to run it on Docker, you can deploy this exporter using the
-[node-exporter Docker
-image](https://quay.io/repository/prometheus/node-exporter) with the following
-options and bind-mounts:
-for docker version >= 1.10
+Be aware that any non-root mount points you want to monitor will need bind-mounted
+into the container.
 ```bash
-docker run -d -p 9100:9100 \
-  -v "/proc:/host/proc:ro" \
-  -v "/sys:/host/sys:ro" \
-  -v "/:/rootfs:ro,rslave" \
+docker run -d \
   --net="host" \
-  quay.io/prometheus/node-exporter \
-    --path.procfs /host/proc \
-    --path.sysfs /host/sys \
-    --collector.filesystem.ignored-mount-points "^/(sys|proc|dev|host|etc)($|/)"
+  --pid="host" \
+  quay.io/prometheus/node-exporter
 ```
-for docker version < 1.10
-```bash
-docker run -d -p 9100:9100 \
-  -v "/proc:/host/proc:ro" \
-  -v "/sys:/host/sys:ro" \
-  -v "/:/rootfs:ro" \
-  --net="host" \
-  quay.io/prometheus/node-exporter \
-    --path.procfs /host/proc \
-    --path.sysfs /host/sys \
-    --collector.filesystem.ignored-mount-points "^/(sys|proc|dev|host|etc)($|/)"
-```
-
-Be aware though that the mountpoint label in various metrics will now have
-`/rootfs` as prefix.
-
 ## Using a third-party repository for RHEL/CentOS/Fedora
 There is a [community-supplied COPR repository](https://copr.fedorainfracloud.org/coprs/ibotty/prometheus-exporters/). It closely follows upstream releases.